The present invention relates to lithium cobalt oxides for use as positive electrode materials for rechargeable lithium and lithium-ion secondary batteries, and to methods of making lithium cobalt oxides.
LiCoO2 is currently being used in over 95% of commercial lithium and lithium-ion secondary batteries as the active positive electrode material. The current production rate of lithium and lithium-ion batteries is about 30 million units per month with each unit containing about 10-15 g of LiCoO2 (i.e., 300-450 metric tons/month).
LiCoO2 can be made by a number of different methods by reacting a lithium salt and a cobalt salt. However, these methods often involve lengthy reaction times thereby reducing the rate of LiCoO2 production.
Therefore, there is a need in the art to provide a method of preparing LiCoO2 that demonstrates good performance in rechargeable lithium and lithium-ion secondary batteries and that can be produced with a relatively short reaction time.
The present invention includes lithium cobalt oxides having hexagonal layered crystal structures and methods of making same. The lithium cobalt oxides of the invention have the formula LiwCo1xe2x88x92xAxO2+y wherein 0.96xe2x89xa6wxe2x89xa61.05, 0xe2x89xa6xxe2x89xa60.05, xe2x88x920.02xe2x89xa6yxe2x89xa60.02 and A is one or more dopants. Preferably, 0.98xe2x89xa6wxe2x89xa61.02 and 0xe2x89xa6xxe2x89xa60.02.
The lithium cobalt oxides of the invention preferably have a position within the principal component space defined by the following relationship:
axi+byixe2x89xa6c 
wherein xi={right arrow over (S)}ixe2x97xaf{right arrow over (P)}c1; yi={right arrow over (S)}ixe2x97xaf{right arrow over (P)}c2; the vector {right arrow over (S)}i is the x-ray spectrum for the LiwCo1xe2x88x92xAxO2+y compound; the vectors {right arrow over (P)}c1 and {right arrow over (P)}c2 are determined by measuring the x-ray powder diffraction values {right arrow over (S)}i between 15xc2x0 and 120xc2x0 using a 0.020 step size and CuKxcex1 rays for a large sample set of lithium cobalt oxides and using the regression of {right arrow over (S)}i of the sample set against the capacity fade after 50 cycles of a lithium coin cell that includes a lithium negative electrode and the lithium cobalt oxide as the positive electrode material and that is cycled between 3.0 and 4.3V at a constant current of C/3 during both charge and discharge cycles; and the values a, b and c are determined by using only the xi and yi values for LiwCo1xe2x88x92xAxO2+y compounds in the sample set that have a capacity fade after 50 cycles of less than or equal to 15%.
More preferably, the lithium cobalt oxides of the invention have a position within the principal component space defined by the following relationship:
xi+0.77yixe2x89xa6xe2x88x926 
wherein xi={right arrow over (S)}ixe2x97xaf{right arrow over (P)}c1; yi={right arrow over (S)}ixe2x97xaf{right arrow over (P)}c2; {right arrow over (S)}i is the x-ray spectrum for the LiwCo1xe2x88x92xAxO2+y compound; and {right arrow over (P)}c1 and {right arrow over (P)}c2 are determined by measuring the x-ray powder diffraction values sh between 15xc2x0 and 120xc2x0 using a 0.020 step size and CuKxcex1 rays for a large sample set of lithium cobalt oxides and using the partial least squares regression (PLSR) of {right arrow over (S)}i of the sample set against the capacity fade after 50 cycles of a lithium coin cell that includes a lithium negative electrode and the lithium cobalt oxide as the positive electrode material and that is cycled between 3.0 and 4.3V at a constant current of C/3 during both charge and discharge cycles. For example, {right arrow over (P)}c1 and {right arrow over (P)}c2 can be defined by the coefficients provided in Table 1 (wherein 2xcex8 is the scattering angle of x-ray powder diffraction measurements using CuKxcex1 rays).
The lithium cobalt oxides of the invention can be used in the positive electrode of a rechargeable lithium or lithium-ion secondary battery in accordance with the invention. For the lithium cobalt oxides of the invention, the capacity fade of a lithium coin cell having a lithium negative electrode and using the lithium cobalt oxide as the positive electrode material when cycled between 3.0 and 4.3V at a constant current of C/3 during both charge and discharge cycles after 50 charge/discharge cycles is preferably less than or equal to 15%, more preferably less than or equal to 10%. Moreover, the initial specific discharge capacity is preferably greater than or equal to 154 mAh/g.
The lithium cobalt oxides of the invention are prepared by heating a lithium source compound, a cobalt source compound and optionally one or more source compounds that include dopants A at a temperature below about 850xc2x0 C. to produce LiwCo1xe2x88x92xAxO2+y and heating the LiwCo1xe2x88x92xAxO2+y compound at a temperature from about 900xc2x0 C. and 1000xc2x0 C. to form and enhance the hexagonal layered crystal structure of the LiwCo1xe2x88x92xAxO2+y compound. The temperature in the first heating step is preferably from about 500xc2x0 C. to about 850xc2x0 C. and the temperature in the second heating step is preferably from about 950xc2x0 C. to about 980xc2x0 C. The source compounds in the first heating step can be heated at more than one temperature below about 850xc2x0 C. In addition, the LiwCo1xe2x88x92xAxO2+y compound in the second heating step can be heated at more than one temperature from about 900xc2x0 C. to about 1000xc2x0 C. Preferably, the first heating step comprises heating the source compounds for a period of time of from about 30 minutes to about 3 hours and the second heating step comprises heating the source compounds for a period of time of from about 30 minutes to about 7 hours. The lithium source compound preferably used with the invention is selected from the group consisting of Li2CO3 and LiOH and the cobalt source compound is preferably selected from the group consisting of Co3O4 and Co(OH)2. The lithium cobalt oxide is preferably cooled after the heating steps at a rate of from 8xc2x0 C./min to 140xc2x0 C./min, more preferably from 10xc2x0 C./min to 100xc2x0 C./min.
The present invention further includes a method of analyzing a compound to determine if the compound is suitable for use as the active positive electrode material in a lithium or lithium-ion secondary battery. The method of the invention comprises determining a principal component space defined by the relationship axi+byixe2x89xa6c, wherein xi={right arrow over (S)}ixe2x97xaf{right arrow over (P)}c1; yi={right arrow over (S)}ixe2x97xaf{right arrow over (P)}c2; the vector {right arrow over (S)}i is the x-ray spectrum for the compound; the vectors {right arrow over (P)}c1 and {right arrow over (P)}c2 are determined by measuring the x-ray powder diffraction values {right arrow over (S)}i for a predetermined range of 2xcex8 values using a predetermined step size by sampling a plurality of samples of compounds having the same general formula as the compound and using the regression of {right arrow over (S)}i of the sample set against predetermined battery performance data for the samples by incorporating the samples as the positive electrode active material in a lithium or lithium-ion secondary battery. The xi and yi values for the compounds in the sample set that have the predetermined battery performance data are then used to determine a, b and c. The compound is analyzed by measuring the x-ray diffraction spectra for the compound to obtain x and y values and determining whether the x and y values meet the relationship axi+byixe2x89xa6c and thus whether the compound possesses the battery performance and x-ray diffraction spectra suitable for use as the active positive electrode material.
The lithium cobalt oxides of the invention have good cycleability properties including good initial specific capacities and capacity fades and thus are desirable for use in rechargeable lithium and lithium-ion batteries. In addition, the lithium cobalt oxides of the present invention can be produced quickly in less than 10 hours and thus can be produced at a rate desired in the art.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description, which describes both the preferred and alternative embodiments of the present invention.